Photographic processes according to which direct positive images can be obtained without requiring reversal processing or negative film are well-known.
As for the method of forming a direct positive image utilizing a silver halide emulsion of internal latent image type, various methods are known, such as those described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,267 and 3,796,577, British Patents 1,151,363, 1,150,553 and 1,011,062.
In order to form a direct positive image, an internal latent image type silver halide photosensitive material is processed with a surface color developer after or while receiving a fogging treatment, and then subjected to bleach processing and fixation processing (or a bleachfix processing). After fixation, the photosensitive material is usually subjected to a washing and/or stabilizing treatment.
In the formation of direct positive image using an optical or chemical fogging method, it takes a long time to complete the photographic processing because of a low developing rate, compared with the formation of ordinary negative images. Therefore, a method of increasing the pH and/or the temperature of a developer used has so far been adopted as a measure for shortening the processing time. However, such a method has a problem that a high pH of the developer generally causes an increase in the minimum image density of the direct positive image formed. In addition, deterioration of the developing agent attributable to aerial oxidation and a lowering of pH caused by adsorption of carbon dioxide gas in the air tend to occur under high pH conditions, and thereby the developing activity of the developer is markedly lowered. Another problem is that the development processing performed under high temperatures causes, in general, a decrease in contrast of the positive image produced, especially in the toe portion thereof.
Other methods for increasing the developing rate in the formation of a direct positive image include the utilization of hydroquinone derivatives (as described in U.S. Pat. No. 3,227,552), and the utilization of mercapto compounds containing a carboxylic or sulfonic acid group (as described in JP-A-60-170843 (the term "JP-A" as used herein means an "unexamined published application")). However, the effects produced by using such compounds are small. Under these circumstances, methods of effectively heightening the maximum density of direct positive images without increasing their minimum densities romain to be found. In particular, a method providing a sufficiently high maximum density even by processing with a low pH developer is desired.
Moreover, increase of the development time in photographic processing of direct positive photosensitive materials of internal latent image type has the problem that the contrast in the toe portion of the produced images tends to be decreased. In this respect, methods of increasing developing rate are desired.
Instant color photography (or color forming material transfer process) can provide a positive image in a short time and here also, a reduction of development time is desired.
Direct positive photosensitive materials further have the deficiency that they are inferior to conventional negative photosensitive materials in sharpness and graininess of the produced images. In particular, this defect becomes remarkable when the oxidation of a developing agent in the developer, a lowering of pH, an increase in bromine ion concentration and so on occur in running processing to deteriorate the developer, or the photosensitive materials are stored for a long time under severe conditions. Accordingly, methods achieving satisfactory sharpness and graininess even by the use of a low pH developer are desired.
A method utilizing an internal latent image type emulsion which contains tabular silver halide grains having a grain thickness less than 0.5 .mu.m, a grain diameter of 0.6 .mu.m or more and an aspect ratio of 8 or more is described in JP-A-58-108528, which brings about an improvement in sharpness and graininess of the produced images. However, the improvement is still insufficient. Also, this method has a small effect on the heightening of the maximum image density without being attended by an increase of the minimum image density, and on an increase in developing rate. Further, an increase in contrast in the toe portion of the image is not obtained by this method.
Japanese Patent Application No. 299155/86 discloses that a silver halide emulsion containing monodisperse, hexagonal, tabular silver halide grains has effects on enhancement of the sensitivity in color sensitization, improvement of the image sharpness and lowering of the intrinsic sensitivity, but does not contain any description of methods for forming direct positive images. In addition, high sensitivity and low re-reversal sensitivity are described in an example of the foregoing patent specification wherein the monodisperse, hexagonal, tabular silver halide grains are applied to color instant photography and the aspect ratio of the grains used is 11.3.